Silicon is a well mastered material due to decades of microelectronic industry development. However, as an indirect band gap semiconductor, it suffers from an extremely low spontaneous emission rate. In the SiNaPS laboratory, we investigated how nanoscale silicon could provide efficient light generation and/or extraction. We worked at the electronic level where silicon electronic states can be modified by quantum scale electron confinement. We also worked at the photonic level where the electromagnetic environment of emitters can be tailored by photonic crystals and optical microcavities. With this strategy, we demonstrated very efficient light emission by coupling intrinsic silicon light emission to low group velocity electromagnetic modes of two dimensional photonic crystals, as well as, by coupling radiative centers like erbium atoms to silicon nanoclusters in ultra-high quality factor whispering gallery modes of toroidal microcavities.

Fig. 1: low group velocity photonic crystal

By carefully designing a 2D photonic crystal, one can tailor its band diagram and create low group velocity or “flat-band” modes. As an example, a flat-band electromagnetic mode in the G point of the band diagram corresponds to a high density of low group velocity modes in the plane of the photonic crystal. Moreover, since this mode in G is above the light line, it means that the light is coupled to the continuum of radiative electromagnetic modes and that the light can be indeed extracted from the silicon layer. In Fig 1, we illustrate the results we obtained with such a photonic crystal by using the electron-hole recombination in a crystalline SOI film as a photonic probe of the photonic crystal states. The photonic crystal is made of triangular lattice of silicon rod obtained by e-beam lithography and reactive ionic etching. A thermal oxidation is made in order to neutralize electronic surface defect states. The period of the lattice (460 nm) and the diameter of si rod (360 nm) are chosen in order to have one flat photonic band allowed in the range of energy of electron hole recombination in silicon (near 1 µm).We observe in the photoluminescence experiment performed at room temperature that a huge peak appears in the photonic crystal spectrum at the frequency of the low group velocity mode. As compared to the reference sample (without photonic crystal), a two orders of magnitude enhancement of the light collected above the sample is observed. FDTD simulations of the Poynting vector confirm that whereas in the reference sample the majority of light is guided in the plane, the light is indeed redirected out of the plane in the photonic crystal structure due to the low group velocity mode.

In Fig 2, we show the 1.5 µm light emission at room temperature collected from a microtoroidal cavity. The microtore is obtained by melting a silica microdisk on silicon pedestal by CO2 laser.Microdisks are fabricated using a combination of optical lithography, wet chemical oxide etching and dry reactive ionic silicon etching. Then a layer of erbium doped silicon rich oxide is deposited by co-evaporation on the top of the toroidal cavity as an emitter. This kind of emitter can be seen like a whole of different size nanoclusters of silicon embedded in erbium doped silica matrix. The small size of nanocluster modify the energy of electron hole pairs, and an efficient coupling between electron-hole pairs and erbium atoms in near neighboring (interaction distance less than 1 nm) of nanoclusters allow the energy transfer to erbium atoms. Then erbium atoms relax to their fundamental states by emitting photons in the 1.45-1.60 nm range. In the photoluminescence spectrum of the 14 µm in diameter microtore (Fig. 2), we observe a series of ultra narrow peaks superimposed to the relatively broad luminescence of erbium. These peaks correspond to the whispering gallery modes of the toroidal resonator, and we observe the relaxation of erbium atoms in these ultra high quality factor modes.The effects shown here suggest that photonic crystals and microcavities may play a critical role in the quest for silicon based efficient micro light sources.